1. Field of the Invention
In recent years, the automotive industry has been giving considerable attention to improved vehicle suspension systems in which the suspension system may be selectively placed in a "soft" ride mode or a "firm" ride mode. When in the soft ride mode, the suspension system functions primarily as a cushioning device which largely isolates the vehicle body from bouncing movement of the wheels induced by small bumps, etc. in the road surface. Effectively, in the soft ride mode, the suspension system imposes a relatively small resistance to relative movement between the sprung and unsprung components of the suspension. While this particular mode provides a very comfortable ride during normally encountered driving conditions, it will accommodate an undesirably large amount of movement of the vehicle body relative to the wheels during high-speed cornering or emergency braking of the vehicle.
When such suspension systems are in their firm ride mode, the system exerts a much firmer resistance to relative motion between the sprung and unsprung components. The handling characteristics of the vehicle are greatly improved in the firm ride mode; however, bumps in the road surface are not absorbed by the suspension system to any great extent.
2. Description of the Related Art
In the recent past, in addition to enabling the vehicle driver to select either the soft or firm mode, as by a selector switch, such systems have employed an electronic processor to automatically shift all, or parts, of the suspension system to the firm mode in response to variations in certain operational parameters monitored by the processor. For example, if the system had been manually set in its soft mode and an emergency stop was required, the front wheel suspensions would be automatically shifted by the processor to their firm mode in response to the sensing, by the processor, of an increase in hydraulic pressure in the brake lines above a preset pressure. This stiffening of the front end suspension elements minimizes "dive"- i.e., the dropping or lowering of the front end of the vehicle in a panic stop situation.
In such systems, control of the relative stiffness of the suspension is accomplished by controlling the size of an orifice associated with a shock absorber or hydraulic suspension strut to control the rate of flow of fluid displaced by compression of the strut or shock absorber. Typically, a solenoid controlled valve is employed for this purpose, the valve in one position defining a relatively large orifice imposing a minimum restriction to flow (soft ride mode) and presenting, in its other position, a relatively small orifice imposing a substantial restriction to flow through the valve (firm ride mode). The processor was simply programmed to set the valve in one position or the other in accordance with whether a particular parameter monitored by the processor was above or below a preselected value. The controlled strut or shock absorber was thus either in a soft mode or a firm mode and its motion damping characteristic when in one mode differed substantially from its damping characteristic when in the other mode. The selection of the parameter value at which switching between modes took place was, to some extent, an educated guess, and the valve on occasion would shift back and forth between its alternative modes when the monitored parameter hovered about the selected shift point.
The present invention is directed to a solenoid actuated valve which may be operated to continuously vary the orifice size or restriction to flow continuously over the range between the extremes represented by the soft and firm mode settings employed in prior art systems.
Present-day electronic processors are readily capable of generating control signals which accurately and precisely track variables in inputs presented to the processor. However, the conversion of these precise electrical signals into a mechanically variable flow restriction is a problem which has not been adequately solved by the prior art. The present invention utilizes a pulse width modulated control signal generated in a known manner by the processor to cyclically deenergize or energize the solenoid coil of a solenoid actuated valve. While the valve of the present invention has but two positions, it is especially designed to be rapidly shifted between these two positions so that the effective restriction presented by the valve over a period of time is a time averaged restriction which is intermediate between the minimum and maximum restrictions represented by the percentage of time within a given cycle that the valve is at one of its two positions.